Catalyst systems comprising lanthanide compounds are known to be useful for polymerizing conjugated dienes. These catalyst systems can be stereospecific and, to some extent, can selectively produce cis-1,4-polydienes or trans-1,4-polydienes, depending on the specific catalyst system. For example, a catalyst system including a lanthanide-containing compound, an alkylating agent, and a halogen-containing compound can be useful in producing cis-1,4-polydienes from various conjugated diene monomers. These catalyst systems are also capable of copolymerizing different types of conjugated diene monomers to give stereoregular cis-1,4-copolydienes.
Cis-1,4-polydienes produced by lanthanide-based catalyst systems have a linear backbone structure and exhibit good green strength and excellent viscoelastic properties. The linear backbone structure is believed to improve the tensile properties and abrasion and fatigue resistance, and to reduce hysteresis loss of rubber compounds. Therefore, these polydienes are particularly suited for use in tire components, such as sidewall and tread. Despite advances, however, catalyst systems to produce improved polydienes are still desired.